This invention relates generally to rotors, and more specifically to methods and apparatus for controlling the rotational speed of a rotor.
During operation of at least some known wind turbines, rotor speed is controlled to prevent the wind turbine rotor from rotating at or above a predetermined speed that may damage components of the wind turbine. For example, at least some known wind turbines control rotor speed by pitching the rotor blades using a pitch drive system that changes an angle of the rotor blades, thereby changing the aerodynamic torque of the rotor. Since their introduction, wind turbines have continuously increased in physical size and electrical power output. However, as rotor diameter and therefore rotor blade lengths increase, friction within the pitch drive system may also increase, which may increase the torque required by the pitch drive motor to pitch the rotor blades. Moreover, as rotor blade lengths increase it may also be more difficult to accurately control the rotor blade pitch because of increased gear backlash within the pitch drive system. As such, the pitch drive system may be required to perform more pitching operations to accurately control the angle of the rotor blades, which may increase parasitic power loses within the wind turbine and/or may increase an amount of cooling required by the pitch drive system to prevent damage thereto and/or failure thereof. Moreover, pitching the rotor blades may increase loads induced to a tower of the wind turbine due to the effect of blade pitch on rotor thrust.